Flexible offshore pipes of the type in question normally comprise a tube-formed inner liner and at least one reinforcement layer. Often such pipes are used for the transportation of oil and gas products over long distances and often at elevated temperatures, such as above 60° C. or more.
Flexible offshore pipes are also used for injection of chemicals into a sub-sea drilled well e.g. connected between a host oil platform and a sub-sea satellite installation.
Flexible pipes may also be used for transportation of other liquids such as water preferably over long distances both onshore and offshore—e.g. for delivering water to dry areas.
A general requirement to such flexible offshore pipes is that they should be capable of operating at relatively high pressures, and the pipes should be sufficiently resistant to the fluids they are transferring and the environment they are operating in. It is thus highly desirable that the flexible pipes are resistant to e.g. chemicals, crude oils, aggressive gasses and/or water, including seawater. Furthermore such flexible pipes should be flexible so that they can be spooled onto a drum or reel. In practice this is the whole point in using flexible pipes.
Flexible pipes e.g. for offshore use are normally very long. Risers, which are pipes extending at least to a certain degree vertically from seabed towards sea surface, are often several hundred meters long and flow-lines, which are pipes extending essentially along the seabed, are often several kilometers long. The flexible pipes are typically subjected to high pressures and pressure differences along the pipeline. When the pipe is transporting oil or gas, the pipes may be exposed to temperatures substantially above 60° C. The flexible pipes should therefore preferably be capable of operating at high temperatures and high pressures.
Offshore pipes generally comprise one or more tube-formed layers including an inner liner, and at least one reinforcing layer. The inner liner is the innermost polymer layer, which in known flexible pipes also constitutes a barrier the fluid to be transported, e.g. crude oil.
In most situations, the pipeline also comprises an outer polymeric sheath providing a barrier to the outer environment such as seawater. The pipe normally comprises one or more reinforcing layers between the inner liner and the outer sheath, and some pipes also comprise a reinforcing layer inside the inner liner, called a carcass. The carcass prevents collapse of the inner liner and provides mechanical protection to the inner liner. Some pipes also comprise one or more intermediate polymer layers.
All of these layers affect the property of the flexible pipe, including the strength, the barrier effects, the flexibility and the durability.
Continuous efforts are applied in order to improve properties of polymer layers of flexible pipes such as for example polymers for the inner liner.
In general it is desired that the inner liner should be chemically stable and mechanically strong even when subjected to high temperatures. A number of polymers are presently used for the production of inner liners as well as other polymer layers of the flexible pipe, such as Polyamide-11 (PA-11), polyethylene (PE) and Polyvinylidene diflouride (PVDF).
These materials are chosen to fulfil the combined requirements in a specific application e.g. heat stability, resistance to crude oil, resistance to seawater, resistance to gases, resistance to acidic components, mechanical fatigue, ductility, strength, durability and processability.